1. Field of the Invention
This invention is related generally to a method and apparatus for thermally treating the exhaust gases of a combustion process by reheating the exhaust gases so as to destroy any residual hazardous compounds contained in the exhaust gases, and then to cool the reheated exhaust gases to reduce the stickiness of particles contained in the exhaust gases. This invention is related specifically to a lengthened and/or enlarged bypass arrangement coupled to a hazardous waste-fueled rotary cement kiln, the bypass comprising one or more chambers which may include afterburners to treat, often by reheating the exhaust gases so as to destroy organic compounds contained in the exhaust gases from the cement kiln and a quench means for cooling the bypassed exhaust gases to prevent particles contained in the bypassed exhaust gases from condensing and/or solidifying on and sticking to the walls of the apparatus.
2. Prior Art
Hazardous waste incinerators use afterburners in special swirl chambers to reheat the incinerator exhaust gases to a minimum of 1800.degree. F. with a retention time of at least two seconds. These swirl chamber arrangements with an afterburner generally are regarded as sufficient to guarantee a minimum of 99.99% destruction efficiency of organic compounds. For example, see Brunner D. R., Handbook of Hazardous Waste Incineration, TAB Professional and Reference Books, Blue Ridge Summit, Pa., 1986.
In all cement kilns, fuel is burned at the hot, discharge end of the kiln. This fuel may consist of coal, oil, or natural gas, or it may include petroleum coke, tires, and hazardous waste fuel as well. In addition, in cement kilns burning hazardous waste fuel, the solid hazardous waste fuel is metered typically in discrete increments into the rotary kiln. In U.S. Pat. No. 3,572,524 to Muckenheim, an apparatus for charging sludges and other similar waste materials to a rotary incinerating kiln is disclosed which charges the waste to the feed end of the kiln. In U.S. Pat. No. 4,850,290 to Benoit et al., a method for charging drums of solid hazardous waste directly into the central portion of a rotary kiln is disclosed. No matter how the waste fuel is charged to the kiln, any fuels, such as discarded tires, and most specifically hazardous waste fuels, can contain organic compounds a portion of which, when incinerated, may be released incompletely combusted in the exhaust gases from the kiln.
For preheater kilns, the feed end of a rotary kiln is the transition between the preheater, or the preheater with precalciner, and the rotary kiln. In these kilns, it is the locale typically used for firing supplemental fuel other than that fired at the hot or discharge end of the kiln. Due to the fact that the raw material kiln feed and any fossil fuel used contain alkali metal (sodium and potassium) compounds, and the hazardous waste fuels typically contain considerable amounts of chlorides and other inorganic materials, the potential exists for the formation of low-melting alkali chloride salts. These salts have the potential to cause the formation of buildups on the exhaust gas ductwork and plug the system. Therefore, most preheater and precalciner kilns in the United States have been equipped with a bypass system to extract some of these sticky alkali chloride/alkali sulfate salts and reduce their concentration in the exhaust gas stream, thereby minimizing the likelihood of the buildups.
Conventional bypasses typically extract 10% or more of the rotary kiln exhaust gases from the riser duct prior to the entry point of the preheated raw meal from the second-lowest preheater stage. The amount of bypass gases extracted from the system depends upon the amount of chlorine, alkali, and sulfate which need to be removed from the pyroprocessing system in order to ensure steady state operations. It is known that chlorides, alkalis and sulfates, unless they are being removed directly from the pyroprocessing system, tend to form buildup in the lower part of the preheater, the riser duct, and in the feed end housing. These buildups occur because the alkali chloride and sulfate vapors and liquid droplets traveling with the hot exhaust gases from the kiln are cooled down by the cooler raw meal particles fed to the kiln, or by the lower temperature refractory wails.
The formation of buildup of alkali chlorides and sulfates also can occur in the bypass takeoff arrangement, as well as in the bypass duct between the takeoff and the bypass quench chamber. The bypass quench chamber serves to cool rapidly the hot kiln gases containing alkali chlorides and sulfates with ambient air, and to condense and solidify these vapors and liquids while they are freely suspended in the gas stream so that they do not stick to the walls and initiate buildups. Typical bypass designs minimize the length of the duct between the bypass takeoff and quench chamber in an effort to minimize the areas on which buildup can occur. This is exemplified in U.S. Pat. No. 3,784,389 to Hastrup. As a result, the retention time of the bypass gases for transport from the kiln riser duct to the quench chamber is rather short. For applications in which a short bypass is sufficient, such short bypasses may reduce the quantity of material condensing and building up on the walls of the apparatus.
Although shorter bypasses may help prevent some buildup of scale on the walls of the apparatus, such a structure is not suitable when the kiln itself does not achieve an adequate degree of destruction of hazardous constituents, such as principal organic hazardous compounds (POHCs). Shortened bypasses may not allow sufficient time for destruction or conversion of such hazardous compounds into innocuous combustion products. Gas temperature and gas retention time are known to be the most important factors for a substantially total destruction of hazardous compounds such as the organic compounds remaining in the exhaust gases produced when hazardous wastes, or any other fuels, are used in combustion apparatuses. These two parameters are especially critical, of course, when hazardous wastes are fed to the feed end of the rotary kiln.
For the kiln exhaust gases that are not extracted by the bypass, the gas residence time at elevated temperatures is typically in the range of 0.5 to 2 seconds before the gases enter the lowest stage cyclone such that considerable additional destruction of the residual organic compounds in the kiln effluent occurs. Thereafter, as the gases pass upwards through each of the remaining riser ducts and cyclones on their way to the air pollution control device, the temperatures are successively lower and lower, such that very little if any additional combustion of residual products of incomplete combustion (PICs) can occur. In the case of a preheater with precalciner, the gas retention time at elevated temperatures above 1900.degree. F. is even longer, because, from a thermal perspective, the precalciner functions similarly to an afterburner found in hazardous waste incinerators. Thus, for the exhaust gases sent through the preheater and precalciner to the pollution control device, the temperature and residence time may be sufficient to destroy or convert nearly all of the POHCs and PICs.
The temperature at the back end of the rotary kiln, in the area of the bypass takeoff and in the riser duct up to the point that the raw meal from the second lowest preheater stage it; discharged into the riser duct typically is in the range of 1900.degree. F. to 2200.degree. F. However, the gas residence time between the point where the hazardous waste fuels are fled and the bypass takeoff is fairly short, typically on the order of 0.4 seconds. For some POHCs, this temperature and residence time combination is not sufficient to completely destroy POHCs or convert them to harmless combustion products. Thus, current bypasses are inadequate for the destruction of these particular POHCs or PICs.
The two primary goals of current bypass technology are to maintain the kiln at steady state during operation and to quickly cool the exhaust gases so as to prevent particles contained in the exhaust gases from condensing on and solidifying on and sticking to the walls of the apparatus to the extent that operations are hampered. With the growing use of hazardous wastes as a fuel for a rotary kiln, it can be seen that there is a need for new and improved bypass technology for achieving the above two goals, while at the same time allowing for the more complete destruction or conversion of any organics contained in the exhaust gases. It is to this end that the present invention is directed.